The way ive just got over this prob, for my two. (3 & 4 year old that is)
Is to use 2 Childs bike seats. That ive heated and moulded to fit over the rear jockey seats. Strap goes through the bike seat and around the jockey cushion. Another strap through the top loop of the jockey and seat.
Fits/ removes in seconds. You can still get into the seat real easy.
Havent trialed it yet, so Im not sure what the verdicts going to be.
Seat like this: http://www.allterraincycles.co.uk/product/101858.html
Beats them tumbling around the deck and into the bilge.